Replace Cisco PIX 515 E with?

How about the ASA 5510 or 5520?

http://www.cisco.com/en/US/products/ps6120/index.html

Depending on what you are using the 515E for, the ASA5505 w/50 user or unlimited licenses may suite your needs and save you some money over the other units.

Based on your initial post, it does not appear that you have redundant devices and as such, I'm inclined to believe the ASA5505 will meet your needs.

This will depends on how many users, are you planning on using VLANS, are you planning on publishing any internal hosts to the public, are you looking for a VPN solution as well?

yes we have a hosted web application and vpn as well

Assuming PIX 515E was suited for your needs I suggest to choose based on the EOL document
http://www.cisco.com/en/US/prod/collateral/vpndevc/ps5708/ps5709/ps2030/end_of_life_notice_for_the_Cisco_PIX_515E_Security_Appliance.html

How many VPN clients and how many site to sites?   Also, do you have a DMZ zone for your public facing system?    If yes, do your internal clients need to access this host as well?

we have around 15 vpn users and 1 site to site vpn, this firewall is hosting center and all are public facing. there is no DMZ, Internal users use site to site VPN to access hosting center.

Depending on how many of those remote vpn users typically connect simultaneously, you may want to consider starting with the base model and adding the sec plus license later if needed.

The base unit will support up to 10 simultaneous IPSec sessions.

The sec plus license essentially also doubles the cost of the unit and since it can be added later, you might want to determine if your remote users are always connected.

Yes - Good point.